If you do this, you should not access
MyISAM tables from another program (such
as from another MySQL server or with
myisamchk) when the tables are in use.
Doing so risks index corruption. Using
--external-locking does not
eliminate this risk.

The maximum size of the temporary file that MySQL is permitted
to use while re-creating a MyISAM index
(during REPAIR TABLE,
ALTER TABLE, or
LOAD DATA
INFILE). If the file size would be larger than this
value, the index is created using the key cache instead, which
is slower. The value is given in bytes.

Automatic recovery is activated if you start
mysqld with the
--myisam-recover-options option. In
this case, when the server opens a MyISAM
table, it checks whether the table is marked as crashed or whether
the open count variable for the table is not 0 and you are running
the server with external locking disabled. If either of these
conditions is true, the following happens:

The server checks the table for errors.

If the server finds an error, it tries to do a fast table
repair (with sorting and without re-creating the data file).

If the repair fails because of an error in the data file (for
example, a duplicate-key error), the server tries again, this
time re-creating the data file.

If the repair still fails, the server tries once more with the
old repair option method (write row by row without sorting).
This method should be able to repair any type of error and has
low disk space requirements.

If the recovery wouldn't be able to recover all rows from
previously completed statements and you didn't specify
FORCE in the value of the
--myisam-recover-options option,
automatic repair aborts with an error message in the error log:

Error: Couldn't repair table: test.g00pages

If you specify FORCE, a warning like this is
written instead:

Warning: Found 344 of 354 rows when repairing ./test/g00pages

If the automatic recovery value includes
BACKUP, the recovery process creates files with
names of the form
tbl_name-datetime.BAK.
You should have a cron script that
automatically moves these files from the database directories to
backup media.